Uranium /jʊˈreɪniəm/ – a radioactive chemical element of group III of the periodic system of Mendeleev, atomic number 92, atomic mass 238.0289, belongs to the actinides. Natural uranium consists of a mixture of three isotopes: 238U (99.282%, T1/2 4.468•109 years), 235U (0.712%, T1/2 0.704•109 years), 234U (0.006%, T1/2 0.244•106 years). There are also 11 known artificial radioactive isotopes of uranium with mass numbers from 227 to 240. 238U and 235U are the founders of two natural decay series, as a result of which they turn into stable isotopes 206Pb and 207Pb, respectively.